The present invention relates to keypads and in particular to a rugged keypad incorporating pressure transducers and processing circuitry whereby threshold levels can be altered to accommodate changes in the physical condition of the keypad such as occur when the faceplate of the keypad is dented, damaged or otherwise distorted.
Keypads used for public telephones, control panels in industrial environments, automatic teller machines and the like are subject to extreme abuse and environmental conditions such as temperature and humidity which can damage the keypad data entry device rendering control of machinery, pay telephone, automatic teller machine and the like impossible using normal keypads. Consequently, there is a need for a keypad with no moving parts which is capable of being entirely encased to seal internal electronic components from environmental conditions. Further, the keypad should be able to utilize materials on a faceplate which are inherently durable such as stainless steel, thick hard coated Lexan or other similar materials substantially immune or significantly resistant to damage. Finally, the keypad should be able to adapt to compensate for any key region on the faceplate which is dented, gouged or distorted thereby changing the non-depressed steady state condition of one or more keys of the keypad.
The adaptability of the keypad is provided by incorporating a pressure transducer at each key and smart electronics which periodically scan the keypad to determine if a new or changed non-depressed state of a key, as indicated by the pressure sensor, has been created by a dent or distortion in the keypad and thereafter modifying the initial or non-depressed value defined for that key. The circuitry then determines whether a key is depressed by intermittently scanning the keypad and comparing the value of the pressure from each pressure sensor for each key against the non-depressed steady state value of pressure for that key. If a predefined criteria is met by the value generated from that comparison, a signal is generated indicating that the key has been depressed.
The keypad of the invention may be configured in either discreet switching elements or a continuous array where each key includes a pressure sensor which is a zero travel device capable of responding to forces applied by fingers to alter the resistance of the pressure sensor or transducer. Electronics incorporated with the invention scan the resistive keypad for changes from a static or rest condition caused by the application of a force such as by a human touch. The force is transferred through a semi-rigid faceplate material to a pressure transducer positioned below the faceplate material. Once a force threshold is surpassed, the electronic circuitry generates a signal indicating the key has been depressed. This signal is supplied to a utilization circuit. Since pressure alone causes key actuation, the keypad responds to pressure applied by gloved hands as well as implements of all types such as screwdrivers, pens and the like.
Keypads according to the invention are durable due to the use of stainless steel or plastic faceplate materials and the electronics which can sense and modify the initial or rest condition thresholds beyond which a "key depressed" indication is given, should the keypad be damaged, dented, or distorted. In effect, the pressure transducers and circuitry allow the keypad to be self-healing after damage to thereby restore the functionality of the keypad.
Keypads as described herein may be sealed and hence are operable even if totally immersed in a liquid or subjected to an environment of harsh chemicals or temperature. Furthermore, since the keypads and associated electronics are low current, passive devices, their operation is unaffected by electromagnetic radiation, vibration or shock. The keypads are thus extremely rugged and intrinsically safe. Furthermore, the keypads are of a very thin construction which can be adhesively attached to any flat surface providing additional operational and functional flexibility.